Explosively releasable bolt



Oct. 11, 1966 F. B. BURKDOLL EXPLOSIVELY RELEASABLE BOLT Filed Aug. 4,1964 INVENTOR. FRANCIS B. BURKDOLL ATTORNEY United States Patent3,277,766 EXPLQSTVELY RELEASABLE BOLT Francis E. Burkdoll, Sunnyvale,Calif., assignor to the United States of America as represented by theUnited States Atomic Energy Commission Filed Aug. 4, 1964, Ser. No.387,539 3 Claims. (Cl. 85-4) The invention described herein was made inthe course of, or under, Contract No. AT(2 9-l)-789, Purchase Order No.82-4499 with the United States Atomic Energy Commission, and saidinvention may be manufactured and used by or for the Government of theUnited States of America without the payment of any royalties thereon ortherefor.

This invention relates to bolts and like fasteners and in particular tofasteners released by the firing of an explosive charge.

Explosive bolt devices of the existing art generally rely on theexcessive pressure of contained gases resulting from ignition of anexplosive charge that burst or fragment the bolt somewhat in the mannerof a hand grenade. Naturally such devices fill the surroundings withflying shrapnel endangering persons and equipment in the vicinity andoften rendering the equipment inoperative. Such an untoward result canbe very costly under circumstances in which such fasteners are oftenemployed, e.g., in space vehicles or the like wherein the fasteners maybe used for securing detachable portions, jettisoning fuel tanks, andthe like.

The present invention utilizes the energy generated by an explosivecharge in a manner not dependent upon contained gas pressure to achievea precisely directed and controlled disruptive separation of a boltstem. in the device of the invention, the arrangement and shape of thebolt stem and the disposition of the explosive charge is especiallyadapted to cause the generation and collision of tensile (dilatationalor rarefaction) shock waves along .a plane in a stress additive fashionso that the tensile strength of the material is exceeded thereat and thebolt separates in that region. It should be noted that by using theabove principle, the energy released in the explosion is, in effect,focused on a small area in order to achieve greater eificiency. Thus asmaller amount of explosive material is needed. Also, with carefullyselected charge sizes little momentum is acquired by the separatedsections and only lightweight restraining guards are needed, at most, toprevent destructive movement of the pieces.

It is, therefore, an object of this invention to provide a devicereleasable upon the discharge of an explosive charge.

It is a further object of this invention to provide a fastening devicereleasable upon the detonation of an explosive charge contained thereinwithout endangering persons or equipment located nearby.

It is a further object of this invention to provide a fastening devicefrangible upon the detonation of an explosive charge contained thereinimparting only low accelerating forces to the separated parts.

It is a further object of this invention to provide a fastening deviceutilizing the colliding shock waves resulting from the detonation of ahigh explosive to cause separation of said fastening device.

It is a further object of this invention to provide an explosivelyreleasable fastening device wherein the explosive energy is efficientlyused thus requiring a minimum amount of explosive material.

Other and more particular objects of this invention will be manifestupon study of the following detailed description when taken togetherwith the accompanying drawing, in which:

Patented Oct. 11, 1966 "ice FIGURE 1 is a longitudinal section throughthe bolt, and

FIGURES 2, 3, 4 and 5 illustrate, sequentially, the propagation of thecompressive and reflected tensile shock wave fronts caused by thedetonation of the high explosive at various times during and afterdetonation, and

FIGURE 6 is a longitudinal section through one type of cartridgecontaining high explosive material arranged to achieve symmetricaldetonation.

Although the description which follows shows an embodiment of thisinvention in the form of a stud, other forms and shapes of fastenerssuch as bolts, rivets, screws or the like could easily be adapted toincorporate this invention by a person of ordinary mechanical ability.

Referring to FIGURE 1 the device of this invention comprises a rod-likestud body 10 having threaded portions 11 and 12 proximate the ends, anexternally tapered section 13 having a sharp edged corner 59 and agenerally cylindrical hole 14 entering from threaded end 12 andterminating in a transverse end wall and having its cylindrical axiscoincident with the longitudinal axis of stud body 10. In thisembodiment, conically tapered section 13 is arranged to slope inwardlyat an angle of approximately to the longitudinal axis of stud body 10toward threaded end 11. This particular slope has been found byexperiment to produce excellent results. It must be noted that thisslope is critically dependent upon detonation properties of theexplosive material used and it determines the plane of collision of thereflected tensile shock Waves and thus the plane of separation orfracture as set forth more fully hereinafter. Hole 14 is arranged toreceive a high. explosive charge 15. Such an explosive charge may beeither pressed in place or be in the form of a cartridge or encapsulatedcombination as illustrated by FIGURE 6 of high explosives and materialsarranged to generate a transverse explosive detonation front proceedinguniformly along the longitudinal length of the explosive charge. Variousexplosive materials such as PETN (pentaerythritoltetranitrate), RDX(cyclotrimethylenetriaminetrinitrate) or the like have been found toproduce excellent results. It must be further noted that for bestresults, hole 14 should be arranged to extend a depth sufficient topermit the bottom of explosive charge 15 to reach or extend slightlybeyond the locus of collision which generally approaches the volumeenclosed by tapered section 13.

Primary explosive detonating means 16 is provided proximate the end ofexplosive charge 15 nearest the opening of hole 14. Care must be takento make sure that detonation of charge 15 is symmetrical, otherwise, theshock waves will not be symmetrical and will not efficiently sever thebolt stem. For this reason, initiation of detonating means 1d must be bya centrally located point source or means similarly purposeful toachieve even and symmetrical initiation of detonation of explosivecharge 15. Wires 17 coupled to a bridge wire (not shown) and disposed inprimary explosive 16 are provided in the instant embodiment as one meansfor electrically initiating detonation although detonating fuse,mechanical strikers, hammers or the like or other means could be usedwithout departing from the concepts and method of operation of thisinvention.

In lieu of a pressed-in-place high explosive charge 15 with primaryexplosive detonating means 16, the detonator cartridge illustrated inFIGURE 6 has been found to produce excellent results. The cartridgecomprises a tubular squib case 101 of copper, steel or similar materialto form a shock Wave impedance match with the bolt material, open at oneend and containing consecutively arranged and abutting, high explosivecharges 201, 202 and 203 to achieve a symmetrical pressure shock wave,which propagates longitudinally. Primary detonation charge 201 comprisesa bridge plug 103 against one end of which, and disposed midway in plug104, is a disk of first detonating explosive material 105. In contactwith explosive material 105 and filling cylindrical plug 104 flush withone end is second detonating explosive material 106. Abutting and incontact with the end of plug 104, containing explosive material 106, issecondary high explosive charge 202 comprising a thick walled cylinder107 of steel, copper or the like, filled with a more powerful highexplosive material 108. Abutting and in contact with the end ofsecondary high explosive charge 202 is tertiary high explosive charge203 comprising a thin walled copper cylinder 109 containing highexplosive material 110. The end of squib case 101 is crimped over bridgeplug 103 to retain and maintain charges 201, 202 and 203 in intimatecontact. Electrical leads 111 enter bridge plug 103 and are connected asto a bridge resistance heater wire, or like means, well known in the artto initiate detonation upon application of electrical current. Uponinitiation, first explosive material 105, preferably, e.g., normal leadstyphnate or the like, is caused to detonate in turn initiatingdetonation in second explosive material 106, preferably lead azide orthe like. The detonation of second explosive material 106 in turninitiates detonation in the more powerful explosive material 108,preferably PETN or the like, confined within the central hole in thickwalled cylinder 107 and having a length several times greater than itsdiameter. During is propagation down explosive material 108, some of theirregularities in the detonating wave front are smoothed out so thatwhen such wave front reaches explosive material 110, it is essentiallyin a transverse plane. The planar detonation wave from explosivematerial 108 then symmetrically initiates detonation in explosivematerial 110, preferably, e.g., RDX or the like, generating asymmetrical plane-transverse shock wave as detonation progresses alongthe length of body of explosive material 110.

In a typical installation a nut 18 or the like, engageable with threadedportion 12 may be used to hold a plate 19 to base plate 20. Base plate20 can be tapped, as shown in this embodiment to receive threadedportion 11 or drilled and arranged to be of a thickness so that a nut,cap-nut or the like can be engaged with the threaded portion 11 of studbody 10.

Care must be taken in the selection of material from which stud body 10is fabricated. It has been found that fragment-free cuts are obtainedover a wide range of explosive charge (146-350 mg. RDX) using SAE 4130steel (cold finished and annealed) and SAE 4620, at room temperature andabove. The body of the bolt in this case had outside diameters of about0.551 inch below and 0.699 inch above the tapered section using a holediameter for the explosive charge of about 0.261 inch. A typical chargewould comprise 6 mg. normal lead styphnate or the like for firstexplosive material 105, 44 mg. lead azide or the like for secondexplosive material 106, 40 mg. PETN for explosive material 108 and146-350 mg. RDX for explosive material 110. The material selected forthe stud body must not be so brittle that the tensile shock wave itself,as it propagates through the material, or the explosive pressure, willcause the ultimate tensile stress of the material to be exceeded toproduce premature and misplaced fracturing. In practice it has beenfound that steels of the type SAE 6150, SAE 1095, highly heat treatedSAE 4130 or the like will badly fragment when employed in the instantinvention. Nor can the material be so soft and ductile that the tensileshock wave is damped to a great extent upon reflection from the outersurface of the stud body and dissipate its energy by plastic flowdeformation rather than separation. In this case, it has been found thatsoft ductile materials such as aluminum generally are unsatisfactory asstud body material for purposes of this invention, although aluminumalloys having mechanical properties approaching those of steel willproduce satisfactory results.

Care must be taken in establishing the diameter of the hole into whichthe bolt stem fits. Since a free boundary, i.e., reflecting surface, isrequired on both sides of sharp edged corner 59, neither of thesesurfaces can be in intimate contact with the material of plates 19 or 20which arrangement would tend to transmit the pressure pulse across thefree boundary and reduce or eliminate the desired reflection from saidsurfaces. Therefore, very loose rather than tight or shrink-fitclearances must be used. In other words, there must not be an impedancematch between the bolt material and the material in contact with thebolt surface in the area of the sharp edged corner. In cases where aloose fit is undesirable, for example, the diameter of the bolt abovetaper 13 can be reduced slightly to provide clearance between the boltand the adjacent material. With such a construction, a resilient orplastic sleeve can be used about the bolt stem to achieve a tight fityet provide a zone of dissimilar impedance, whereby effective reflectionfrom the bolt surface is obtained.

The operation of the device of this invention can be seen through thesequence of events leading to the fracture of the stud body depicted inFIGURES 2, 3, 4, and 5. The length of the high explosive charge 15 hasbeen purposely exaggerated to more clearly display the propagation ofthe shock waves.

Referring to FIGURE 2, a pressure shock wave 51 is generated upondetonation of explosive charge 15 beginning at point 50 and propagatingoutward at an angle a toward outer surface 57. Angle a is determined byvectorially adding the rate of propagation of the detonating wave alonghigh explosive charge 15 to the rate of propagation of the shock wavethrough the stud body material in accordance with the equation:

sine in where C=pressure pulse velocity in bolt material, andD=detonation velocity of the explosive material.

For example, in the case using RDX, the detonation velocity is about 7.8mm./microsec. and for steel, the shock wave velocity is about 5.9mm./microsec. The resulting angle is about 40 degrees. The higheststresses in this case were achieved when 6: 120.

Referring to FIGURE 3 pressure shock wave 51 has reached outer surface57. There results upon reflection of pressure shock wave 51 an inwardpropagating tensile (dilatational or rarefaction) shock wave front 52due to the elastic nature of the stud material. As in the case of amirror, the angle of reflection is equal to the angle of incidence ofthe wave front so that wave front 52 makes an angle on with outersurface 57.

Referring to FIGURE 4 pressure shock wave 51 is now closing in on corner59. A second inward propagating tensile (dilatational or rarefaction)shock wave 53 is therewith reflected from tapered section 13. It hasbeen found that best results are achieved when the angle 0 at corner 59is in the range of -140. It can be seen that angle 0 will determine theangle of collision of the two reflected tensile shock waves, which inturn, by vectorial addition, determines the tensile stresses to be foundat the point of collision.

Referring to FIGURE 5, tensile shock waves 52 and 53 are in collisionalong line 54. Where the tensile stresses in one shock wave alone willbe insufficient to exceed the ultimate strength of the stud material,the vectorial sum of the tensile forces upon collision is made to exceedsaid ultimate strength thus rending apart the stud body from corner 57along a surface or plane defined by the rotation of line 54 about thelongitudinal axis of stud body 10.

Although the foregoing embodiment has been described in detail, thereare obviously many other embodiments and variations in configurationwhich can be made by a person skilled in the art without departing fromthe spirit, scope or principle of this invention. Therefore, thisinvention is not to be limited except in accordance with scope of theappended claims.

What is claimed is:

1. A device for fastening components in contiguous relation andreleasable upon detonation of a high explosive comprising in combinationan elongated rod body having first and second ends adapted to receivemeans for securing said components in loose fitting relation along acentral body area between said ends, said body area including a firstgenerally cylindrical shock wave reflecting surface longitudinallyinwards of said first body end,

' a second shock wave reflecting surface defining a substantiallyfrusto-conical surface converging radially inwardly from saidcylindrical surface toward said second body end with the junction ofsaid first and second reflecting surfaces forming an interior angletherebetween of substantially 120 to define a sharp-edged peripheralcorner, said second reflecting surface joining said first reflectingsurface and a second generally cylindrical portion of reduced diameter,and said body defining a gen erally axial cavity extending from anopening at the first end of said body and extending in generally concentric symmetrical relation to said central body portion to terminateslightly beyond the second end terminus of said second convergingreflecting surface, high explosive means disposed in said central cavitycomprising a detonation initiating means, a planar detonation wavegenerating means and a symmetrical shock wave generating means includinga high explosive coupled for ignition by said planar detonation wavegenerating means, said high explosive means adapted upon initiation toprovide a transverse detonation wave front which progresseslongitudinally therealong to generate a symmetrical compressive shockwave propagating and diverging outwardly from the surface defining saidcavity in said body, to be reflected inwardly by said first and secondsurface areas of said body as tensional shock waves colliding along asurface converging inwardly toward said first body end from said corner,whereat said body is fractured to release said fastening device.

2. In a mechanical fastening device releasable upon detonation of a highexplosive material, the combination in accordance with claim 1 whereinsaid detonation initiating means comprises lead styphnate and leadazide, said planar detonation wave generating means comprises a narrowconfined column of RDX and said shock wave generating means comprisesPETN with said PETN disposed adjacent the closed inner end of said axialcavity.

3. In a mechanical fastening device releasable upon detonation of a highexplosive material, the combination in accordance with claim 1 whereinsaid high explosive charge has a minimum detonation velocity of 5mm./microsec.

References Cited by the Examiner UNITED STATES PATENTS 2,653,504 9/1953Smith -1 2,767,655 10/1956 Seavey 102-28 2,883,910 4/1959 Nessler 85l2,999,460 9/1961 Stinger et al. 102--28 3,158,097 11/1964 Brockway etal. 102--28 3,196,746 7/1965 Dahl 85-1 CARL W. TOMLIN, Primary Examiner.

R. S. BRITTS, Assistant Examiner.

1. A DEVICE FOR FASTENING COMPONENTS IN CONTIGUOUS RELATION AND RELEASABLE UPON DETONATION OF A HIGH EXPLOSIVE COMPRISING IN COMBINATION AN ELONGATED ROD BODY HAVING FIRST AND SECOND ENDS ADAPTED TO RECEIVE MEANS FOR SECURING SAID COMPONENTS IN LOOSE FITTING RELATION ALONG A CENTRAL BODY AREA BETWEEN SAID ENDS, SAID BODY AREA INCLUDING A FIRST GENERALLY CYLINDRICAL SHOCK WAVE REFLECING SURFACE LONGITUDINALLY INWARDS OF SAID FIRST BODY END, A SECOND SHOCK WAVE REFLECTING SURFACE DEFINING A SUBSTANTIALLY FRUSTO-CONICAL SURFACE CONVERGING RADIALLY INWARDLY FROM SAID CYLINDRICAL SURFACE TOWARD SAID SECOND BODY END WITH THE JUNCTION OF SAID FIRST AND SECOND REFLECTING SURFACES FORMING AN INTERIOR ANGLE THEREBETWEEN OF SUBSTANTIALLY 120* TO DEFINE A SHARP-EDGED PERIPHERAL CORNER, SAID SECOND REFLECTING SURFACE JOINING SID FIRST REFLECTING SURFACE AND A SECOND GENERALLY CYLINDRICAL PORTION OF REDUCED DIAMETER, AND SAID BODY DEFINING A GENERALLY AXIAL CAVITY EXTENDING FROM AN OPENING AT THE FIRST END OF SAID BODY AND EXTENDING IN GENERALLY CONCENTRIC SYMMETRICAL RELATION TO SAID CENTRAL BODY PORTION TO TERMINATE SLIGHTLY BEYOND THE SECOND END TERMINUS OF SAID SECOND CONVERGING REFLECTING SURFACE, HIGH EXPLOSIVE MEANS DISPOSED IN SAID CENTRAL CAVITY COMPRISING A DETONATION INITIATING MEANS, A PLANAR DETONATION WAVE GENERATING MEANS AND A SYMMETRICAL SHOCK WAVE GENERATING MEANS INCLUDING A HIGH EXPLOSIVE COUPLED FOR IGNITION BY SAID PLANAR DETONATION WAVE GENERATING MEANS, SAID HIGH EXPLOSIVE MEANS ADAPTED UPON INITIATION TO PROVIDE A TRANSVERSE DETONATION WAVE FRONT WHICH PROGRESSES LONGITUDINALLY THEREALONG TO GENERATE A SYMMETRICAL COMPRESSIVE SHOCK WAVE PROPAGATING AND DIVERGING OUTWARDLY FROM THE SURFACE DEFINING SAID CAVITY IN SAID BODY, TO BE REFLECTED INWARDLY BY SAID FIRST AND SECOND SURFACE AREAS OF SAID BODY AS TENSIONAL SHOCK WAVES COLLIDING ALONG A SURFACE CONVERGING INWARDLY TOWARD SAID FIRST BODY END FROM SAID CORNER, WHEREAT SAID BODY IS FRACTURED TO RELEASE SAID FASTENING DEVICE. 